LA TEE DE VEL OPMENT. 



151 



pleure). The inner wall, or splanchnic layer (spl.m] unites with 

 the entoblast to constitute the wall of the alimentary canal 

 (splanchnopleure). An ingrowth of ectoblast (stomodceum) takes 

 place into the blastopore to form the pharynx, and a similar in- 

 growth at the opposite extremity (proctoclmuni) unites with the 

 blind end of the archenteron to form the anus and terminal part 

 of the intestine. 



As to its origin, therefore, the alimentary canal consists of 

 three portions, viz., (1) the archenteron, consisting of the original 



s.iv 



FIG. 86. Diagram of a cross-section of Lumbricus, showing the relation of the various 

 organs, etc., to the germ-las'ers. Ectoblastic structures shaded with fine parallel lines, 

 entoblastic with coarser parallel lines, mesoblastic with cross-lines; al.c, alimentary 

 canals; ch. chloragogue layer; cce, coelom; c.m, circular muscles of body-wall; c.m.a, 

 circular muscles of alimentary wall; ep, lining epithelium of alimentary canal; rf.r, 

 dorsal vessel; hy. hypodermis or skin; l.m, longitudinal muscles of body-wall; l.m.a, 

 longitudinal muscles of alimentary wall; u, central part of nerve-cord; np, nephridium: 

 is, sheath of nerve-cord; p.e, peritoneal epithelium; r, reproductive organs; s.i.v, sub- 

 intestinal vessel. 



entoblast ; (2) the stomodaeum or pharyngeal region, lined by 

 ectoblast ; and (3) the proctodaeum or hindmost part, also lined 

 by ectoblast. These three parts are called the fo/'c-ijut (sto- 

 modseum), -mid-gut or mesenteron (archenteron), and hind-gut 

 (proctodsBTim), and it is a remarkable fact that these same parts 

 can be distinguished in all higher animals, not excepting man. 



The body now becomes jointed by the appearance of trans- 

 verse folds opposite the dissepiments, and the metamerism of the 

 body becomes evident on the exterior. The young worm has 

 thus reached a stage (n, Fig. 85) where its resemblance to the 



